Separation of olefins is an important process in the chemical and petrochemical industry. Currently, the separation is usually done by cryogenic distillation.
However, this technology is energy and capital intensive considering the large volume of olefins produced every year. As a result, alternative technologies have been actively sought.
There are systems reported that use solutions of metal salts capable of reversibly binding olefins. For example, water-soluble copper and silver salts have been reported to bind olefins reversibly in such systems. However, these metal ions are poisoned by contaminants such as acetylene, carbon monoxide, and hydrogen sulfide, which are commonly present in olefin streams.
Thus, it is highly desirable to develop improved reversible binding reagents for use in processes for separating olefins from complex mixtures containing them.
Some transition metal sulfur compounds have been reported to reversibly bind olefins, For example, Shibahara (Inorg. Chim. Acta, Vol. 251, 207-225 (1996)) and Hidai (J.A.C.S., Vol. 116, 3389-98 (1994)) independently reported that mixed-metal sulfur cubane-type compounds react with olefins reversibly. However, in these compounds, the olefin-binding site is at the metal and therefore is likely sensitive to contaminants such as C.sub.2 H.sub.2, CO and H.sub.2 S. In a recent patent application (Amoco, WO 98/16490), similar compounds were disclosed for use in olefin recovery by reversible olefin complexation in an aqueous solution. As expected, the system is adversely affected by CO and H.sub.2 S that are present in the hydrocarbon stream.
DuBois et al., discovered that certain dinuclear molybdenum-sulfur complexes are capable of binding olefins reversibly (J.A.C.S., Vol. 105, 5329-37 (1983); U.S. Pat. Nos. 5,391,791; 5,414,194; and 5,430,225). The dinuclear compounds are disclosed to act as complexing agents for olefms wherein the sulfur ligands in the complexes act as the site of olefm binding. However, the complexes also react with H.sub.2 and with C.sub.2 H.sub.2 and therefore are not suitable for crude feed. Furthermore, the olefin binding energy is quite strong and, as a result, olefins do not readily dissociate.
Schrauzer et al., (J.A.C.S., Vol. 87 (7), 1483-9 (1965)) disclosed the reaction of metal dithiolene complexes with certain olefins. However, the reactions were carried out at reflux conditions (i.e., high temperature) and the olefins used, e.g., norbornadiene, are more reactive than simple olefins. More importantly, there was no suggestion that less reactive olefins such as ethylene and propylene would be similarly reactive or that the reaction would be reversible under mild conditions.
Wing et al., (J.A.C.S., Vol. 92, 1935-9 (1970) and J. Organomet. Chem., Vol. 63, 441-450 (1973)) teach that certain conjugated dienes reacted with bis[1,2-bis(trifluoromethyl)ethylene-1,2-dithiolato]nickel to form olefin adducts. However, these olefins are also highly reactive, and, in some cases, light was required to force dissociation of olefins from the adducts formed. There is no suggestion of reversibility under mild conditions or any expectation of reactivity with simple (mono)olefins.
Krespan (U.S. Pat. No. 3,052,691; J.A.C.S., Vol. 82, 1515-16 (1960); and J.A.C.S., Vol. 83, 3434-37 (1961)) disclosed the synthesis of bis(trifluoromethyl)-1,2-dithiete ((CF.sub.3).sub.2 C.sub.2 S.sub.2). The dithiete is not stable at room temperature and tends to oligomerize to form dimers and possibly higher oligomers. Although the dithiete monomer and dimer (and possibly higher oligomers) have the same chemical composition, they are different compounds. At higher temperatures, the equilibrium between the dithiete monomer and dimer favors the monomer.
U.S. Pat. No. 3,361,777 teaches the preparation of bis[1,2-bis(tri- fluoromethyl)ethylene-1,2-dithiolato]nickel (Ni[S.sub.2 C.sub.2 (CF.sub.3).sub.2 ].sub.2) by treating nickel turnings with bis(trifluoromethyl)-1,2-dithiete. As used therein and as known to one skilled in the art "dithiete" refers to the (CF.sub.3).sub.2 C.sub.2 S.sub.2 monomer only, not a mixture of the dithiete monomer with dimer and higher oligomers.
None of the references regarding transition metal dithiolene complexes teaches or suggests a process in which reversible binding of simple olefins (e.g., C.sub.2 to C.sub.6 olefins) occurs. What is desired is a process by which olefins may be reversibly bound to a compound or complexing agent that is tolerant to contaminants and poisons typically present in olefin-containing streams.
Additionally, it would be desirable to have a process for recovering olefins, particularly low molecular weight olefins from streams containing these olefins, as well as other hydrocarbons and contaminants in which the complexing agent reversibly binds the olefin to be recovered. Moreover, it would be desirable to have a method for synthesizing the nickel dithiolene complex that obviates the need to use pure dithiete and hazardous nickel carbonyl. Applicants' invention addresses these needs.